Water clarity is a measure of how much light penetrates though the water column. Water clarity is dependent upon the amount of particles (e.g. suspended sediment and plankton) and colored organic matter present. Water clarity plays an important role in determining bay grasses and phytoplankton distribution and abundance.

Chesapeake Bay - Indicator Details:

Threshold Map

This map shows how often water clarity was above or below threshold concentrations from March to November 2009. Good water quality is characterized by having water clarity that is frequently above the threshold (shown in shades of green), while poor water quality tends to have water clarity that is frequently below the threshold (shown in shades of orange and red). In 2009, water clarity was below the threshold level for the majority of regions of the Bay; however improvement was seen from 2008 patterns, especially in the Mid and Upper Bay regions. This is an encouraging sign, given that there has been a trend of declining water clarity throughout the Bay over the past 20 years.

Creating this mapWater clarity was measured at approximately 144 stations up to 14 times during the periods of interest (April to October in the tidal fresh, oligohaline and mesohaline regions, and March to November in the polyhaline regions). Secchi depth is the water depth (in meters) that you can just differentiate black and white quarters of a Secchi disc lowered into the water. The proportion of time that water clarity was below the threshold at each station was calculated and then interpolated to provide estimates between the stations.

Data Map

This map shows average water clarity (Secchi depth in meters) from March to November 2009. All Chesapeake Bay tributaries exhibited a gradient in water clarity, with murkier water (shallowest Secchi depth—orange-pink shades) in the mid to upper reaches and clearer waters in the lower reaches (deeper Secchi depths—shades of blue). Lower water clarity in the mid to upper reaches of the tributaries can, in part, be attributed to the mixing of fresh and saline waters leading to a phenomenon known as a turbidity maximum. The Bay's mainstem had a similar pattern, with murkier water in the Upper Bay and clearer water in the Mid and Lower Bay regions. The overall bay-wide patterns of water clarity in 2009 were similar to those seen in 2008 and 2007, although improvement was seen in the lower mainstem regions.

Understanding this mapWater clarity was measured at approximately 144 stations up to 14 times during the periods of interest (April to October in the tidal fresh, oligohaline and mesohaline regions, and March to November in the polyhaline regions). Secchi depth is the water depth (in meters) that you can just differentiate black and white quarters of a Secchi disc lowered into the water. Average water clarity (Secchi depth) at each station was calculated and levels between stations were estimated using spatial interpolation software.

Ranking

Comparison of Bay Health Index scores for 2009 () compared to ()

Score (%)

020406080100

Mid Bay

Choptank River

Upper Bay

Overall Bay

Lower Eastern Shore (Tangier)

Rappahannock River

Potomac River

Upper Eastern Shore

Lower Bay

Patuxent River

Upper Western Shore

James River

York River

Elizabeth River

Lower Western Shore (MD)

0

Patapsco and Back Rivers

0

This figure ranks each region from best to worst water clarity scores for 2009. This figure shows that all but one region score poorly to very poorly for water clarity, and are still a long way from reaching threshold levels. There have been slight improvements in water clarity in the Bay over the last 3 years, however, there is a long-term (23 years) declining trend overall. The Mid Bay and Choptank River regions had the highest water clarity score in 2009, while the Lower Western Shore and the Patapsco and Back Rivers had the lowest.

Additional Info

The EcoCheck report card has three main sections: Overview, Region Summaries, and Indicators/Indices.

Click the next (>>) button above to get a guided tour of some of the key features of the report card card website.

This map shows the Health Index grades for all the reporting regions, but mousing over each region also provides a text summary and clicking will direct the user to the region summary page with more details about the health of that region.

The region rankings table provides an easy way to visualize the Water Quality Index (WQI), Biotic Index (BI), and overall Bay Health Index (BHI) for all regions. Once again, mousing over provides extra information - try it yourself and see what is displayed. Region names, and indicator icons also serve as navigation links.

The comparison graph allow users to compare the current report card to previous year's scores. Mousing over provides actual percentages and bars provide a direct link to the region's summary page.

Now you're at the region summary page for the Overall Bay. This page details the scores for all the individual indicators, as well as a dynamically generated conceptual diagram (please scroll down the page to view). Even this diagram has more information onmouseover, and has direct links to the detailed indicator pages. There is also a vector (SVG) version of the diagram available for download so you can edit this in your favorite illustration software.

The trends graph shows all the indicators and indices for the selected region since 1986. You can dynamically turn on and off each of the indicators to show only the data you are most interested in. Mousing over the graph shows percentage scores and you can zoom in for finer detail in certain time periods. There is also a fully editable vector (SVG) version of the graph available for download, along with an Excel spreadsheet of the data.

This is the threshold map for Chlorophyll a. This map is zoomable and panable and also available in fully editable vector (SVG) and JPG versions.

The trends graph on the indicator pages shows the indicator and indices data for each region since 1986. By default, only the Overall Bay region is displayed, but you can turn on as many of the other regions as you wish.

The watershed stream health page has several tabs, but the key one is the interactive Google maps version of the data.

Please wait while the map loads and then test out the ratings toggle options and the "zoom to area" functionality (click on the magnifying glass and then click and drag to select the are to zoom to).